Gas sterilization

ABSTRACT

A method of sterilising objects including the operations of activating a gaseous medium, ensuring that the activated medium is substantially free of charged species and exposing the object to be sterilised to the activated gaseous medium for a period sufficient to ensure that the object is sterilised.

The present invention relates to the sterilisation of medical and otherequipment by means of chemically active gaseous media.

A proposed technique for sterilising medical equipment, and otherobjects, is to expose them to the action of a plasma. Plasmas in thiscontext are gaseous media which contain a significant proportion ofionised species and free electrons. Examples of methods of carrying outthis technique are disclosed in U.S. Pat. Nos. 3,383,163; 3,851,436;3,948,601; 4,207,286; 4,321,232; 4,348,357; 4,643,876; JapaneseApplication Disclosure numbers 103460/83; 162276/83 and European PatentApplication number 387022A. However, gas plasmas, although effective assterilising agents have been found often to be too chemicallyaggressive, causing damage to an object being sterilised. This failinghas been an inhibiting factor in the general adoption of the technique.

It is an object of the present invention to provide a method ofsterilising objects by means of activated gaseous media, that is to say,gaseous media which contain significant numbers of free radicals,metastable and electronically excited species, but which do not containsignificant amounts of ionised species.

According to the present invention there is provided a method ofsterilising articles by exposing them to a biologically active gaseousmedium, wherein there is included the operations of activating a gaseousmedium to provide free radicals, and/or electronicaly and/orvibrationally excited species, ensuring that the activated gaseousmedium is substantially free of charged species and exposing the articleto be sterilised to the charged species free activated gaseous mediumfor a period sufficient to ensure that the article is sterilised.

Ionised species produced in the activation process are allowed torecombine before reaching the sterilisation chamber so that only theneutral activated gas is applied to the object to be sterilised.

The dissociation and/or electronic excitation of the gaseous medium canbe achieved by means of bombardment with energetic particles, theapplication of DC and varying electric fields, chemically, orphoto-electrically by means of electromagnetic radiation including bothc.w. and high power pulsed RF and microwaves and laser radiation.

Short pulse, high power microwaves produced at a high repetitionfrequency are particularly effective in achieving significantdissociation whilst minimising thermal effects.

Preferably, the gaseous medium includes activating agents which increasethe population of the activated species in the afterglow from theactivation process. Suitable activating agents are: SF₆ ; H₂ O; O₂ ; H₂S; CO; C₂ H₂ ; CH₄ ; Hg; NH₃ ; Cl₂ ; N₂ O; NO; C₂ H₆ or mixturesthereof.

An activating agent can work by enhancing dissociation, enhancing orinhibiting recombination or by surface modification in a dischargechamber. It can form a component of the activated gas and can be addedbefore or after the activating discharge.

A suitable gaseous medium for use in carrying out the present inventioncomprises 41%-89% by volume oxygen with the balance made up by argon,helium or nitrogen, or mixtures thereof and/or up to 9% by volume of anactivating agent which can be H₂ O; N₂ O; H₂ ; N₂ ; NH₃ or NO, ormixtures thereof.

A second suitable gaseous medium for carrying out the present inventioncomprises 30%-99.9% nitrogen with 0.1%-9% by volume of an activatingagent which can be SF₆ ; H₂ O; Cl₂ ; NO; O₂ ; H; CO₂ ; CO; C₂ H₂ ; C₂ H₆; CH₄ ; NH₃ ; NF₃ or mixtures thereof, the balance being made up byargon or helium.

A third suitable gaseous medium for carrying out the invention comprises1%-99%, oxygen with 0.1%-9% by volume of an activating agent such as H₂O; N₂ O; NH₃, H₂, N₂ or NO, the balance being made up by argon orhelium.

A fourth suitable gaseous medium is ammonia (NH₃) containing up to 9% byvolume of an activating agent such as N₂ ; NO; N₂ O; H₂ ; H₂ O.

Other gas mixtures which can be used to carry out the invention are:

N₂ with 0.5-10% O₂ by volume

N₂ with 0.5-30% NO₂ by volume

N₂ with 0.5-30% NO by volume

Ar with 0.5-30% NO₂ by volume

Ar with 0.5-30% NO by volume

Preferably, the excitation of the gaseous medium is carried out at apressure in the range 0.1 to 50 mbar.

The activating agents may be added before or after the main gas isinitially excited. In particular, when NO is the activating agent it isadded after the initial excitation.

One method of sterilising bodies by means of the present invention willnow be described, by way of example, with reference to the accompanyingdrawings, in which

FIG. 1 shows, diagrammatically an apparatus in which the invention maybe carried out and

FIG. 2 is a cross-section of a portion of the apparatus of FIG. 1.

Referring to FIG. 1 of the drawings, gases to make up a gaseous mixtureto be activated and used to sterilise equipment are supplied fromreservoirs 1 in appropriate proportions, mixed and passed into adischarge tube 2 made of quartz or a ceramic material. The dischargetube 2 is surrounded by two cooling collars 3. A plasma ignition coil 4is connected to a probe in contact with the wall of the discharge tube2. The gaseous medium to be excited passes via the discharge tube 2through a section of microwave waveguide 5 to one end of which there isconnected a tuning short circuit 6 and to the other end of which thereis connected a suitable power supply 7. Having been excited, the gaseousmedium passes, via a water-cooled vacuum feedthrough 8 into a stainlesssteel sealed sterilisation chamber 9 in which the sterilisation processis carried out. The sterilisation operation can be observed by means ofa viewing port which forms part of an access door 10 to thesterilisation chamber 9. Pressure in the sterilisation chamber 9 ismaintained at a desired sub-atmospheric level by means of an exhaustconnection 11, vacuum valve 13 and vacuum pump 14. The pressure withinthe sterilisation chamber 9 is measured by means of a vacuum gauge 12.

Referring to FIG. 2, in which those components which are common to bothfigures have the same reference numbers, inside the sterilisationchamber 9 there are a gas baffle 15, a thermocouple 16 and a stand 17for objects to be sterilised.

In one method of sterilisation embodying the invention, nitrogen dopedwith 0.2% by volume of SF₆ /O₂ as an activating agent was passed at aflow-rate of between 300 and 500 standard cc's per minute and a pressurebetween 1 and 7 mbar through an electric field in the waveguide 5produced by microwave power between 500-800 watts.

Another method of sterilisation embodying the invention utilises a gasmixture comprising nitrogen doped with 3% by volume of oxygen as anactivating agent. The gas mixture was passed through the discharge tube2 at a flow rate of 1000 sccm at a pressure of 13 mbar. The dischargetube 2 passed through the section of waveguide 5 as before, but themicrowave power was pulsed at a peak power of 250 kw at a repetitionrate of 600 pulses per second and a duty cycle of 0.06%.

Any charged species produced by the discharge were removed from thegaseous medium by ensuring that the length of the manifold 2 between thewaveguide 5 and the sterilisation chamber 9 was such that, incombination with flow restrictors (not shown) inserted in it,substantially all the charged species would have recombined.

Microscope slides (not shown) contaminated with Escherichia coli andBacillus substilus were exposed to the activated nitrogen together withsulphur hexafluoride for a period of 10 minutes during which theirtemperature did not exceed 60° C., as shown in the following table.Subsequent bacteriological examination showed that the slides had beenfully sterilised.

Other gas mixtures and operating conditions for carrying out theinvention are:

    ______________________________________    N.sub.2 O.sub.2    N.sub.2 flowrate        1.5 sl/m    O.sub.2 flowrate        55 scc/m    microwave power         500 W    Total gas pressure      2 mbar    Peak temperature        60° C.    N.sub.2 /N.sub.2 O    N.sub.2 flowrate        1.5 sl/m    N.sub.2 O flowrate      124 sc/m    microwave power         500 W    Total gas pressure      2 mbar    Peak temperature        43° C.    N.sub.2 /NO    N.sub.2 flowrate        1.55 sl/m    NO flowrate             23 sc/m    microwave power         500 W    Total gas pressure      2 mbar    Peak temperature        55° C.    ______________________________________

In the last case, the nitric oxide is added after the initial excitationof the nitrogen.

    __________________________________________________________________________    EFFECTIVE EXPERIMENTAL REGIMES                Flow Rate                       Flow Rate of                of Active Gas                       Activation Agent                (Standard                       (Standard      Exposure                                           Peak    Active          Activation                c.c per                       c.c per  Microwave                                      Time Temp                                               Pressure    Gas   Agent minute)                       minute)  Power (Mins)                                           (0° C.                                               (Millibar)    __________________________________________________________________________      N.sub.2   300 sscm        500 W 10 min                                           35° C.                                               2 mbar      N.sub.2          SF.sub.6                300 sscm                       2 sscm   500 W 10 min                                           35° C.                                               2 mbar      N.sub.2          SF.sub.6                300 sscm                       0.5 sscm 500 W 10 min                                           36° C.                                               6 mbar      N.sub.2          O.sub.2                1000 sscm                       4 sscm   Pulsed                                      30 min                                           60° C.                                               13 mbar                                250 Kw                                peak    __________________________________________________________________________

We claim:
 1. A method of sterilising articles by exposing them to abiologically active gaseous medium, comprising the operations ofactivating a gaseous medium to provide free radicals, and/orelectronically and/or vibrationally excited species, ensuring that theactivated gaseous medium is substantially free of electrically chargedspecies, and exposing the article to be sterilised to only thecharged-species-free activated gaseous medium for a period sufficient toensure that the article is sterilised.
 2. A method according to claim 1wherein the activating of the gaseous medium comprises the dissociationand/or electronic excitation of the gaseous species achieved by means ofbombardment of the gaseous species with energetic particles, theapplication of DC and varying electric fields, chemically, or by meansof electromagnetic radiation.
 3. A method according to claim 2 where thedissociation and/or excitation is achieved by means of mocrowave or RFradiation.
 4. A method according to claim 2 wherein the electromagneticradiation is in the form of a laser beam.
 5. A method according to claim1 wherein the gaseous medium includes activating agents adapted toenhance the production of free radicals and maximise the effectivelifetime of the activated species.
 6. A method according to claim 5wherein the activating agent is selected from the group consisting ofSF₆ ; H₂ O; O₂ ; H₂ S; CO; C₂ H₂ ; Hg; NO; Cl₂ ; N₂ O; C₂ H₆ or mixturesthereof.
 7. A method according to claim 1 wherein the gaseous mediumcomprises 41%-89% by volume oxygen with the balance made up by gasselected from the group consisting of argon, helium or nitrogen ormixtures thereof.
 8. A method according to claim 7 wherein the gaseousmedium includes up to 9% by volume of an activating agent selected fromthe group consisting of H₂ O; N₂ O; H₂ ; NO or mixtures thereof.
 9. Amethod according to claim 1 wherein the gaseous medium comprises oxygenincluding up to 9% by volume of an activating agent selected from thegroup consisting of H₂ O; N₂ O; H₂ ; N₂ ; NO or mixtures thereof.
 10. Amethod according to claim 1 wherein the gaseous medium comprises30%-99.9% by volume nitrogen with 0.1%-9% by volume of an activatingagent selected from the group consisting of SF₆ ; H₂ O; Cl₂ ; NO; O₂ ;H₂ ; CO₂ ; CO; C₂ H₆ ; CH₄ ; NF₃ or mixtures thereof.
 11. A methodaccording to claim 1 wherein the gaseous medium comprises 1%-99% byvolume oxygen with 0.1%-9% by volume of an activating agent selectedfrom the group consisting of H₂ O; N₂ O or NO and the balance made up ofargon or helium.
 12. A method according to claim 1 wherein the gaseousmedium comprises ammonia (NH₃) containing up to 9% by volume of anactivating agent selected from the group consisting of N₂ ; NO; H₂ ; H₂O.
 13. A method according to claim 6 wherein the gaseous mediumcomprises nitrogen with 0.5-10% O₂, by volume.
 14. A method according toclaim 6 wherein the gaseous medium comprises nitrogen with 0.5-30% N₂ O,by volume.
 15. A method according to claim 6 wherein the gaseous mixturecomprises nitrogen with 0.5-30% NO, by volume.
 16. A method according toclaim 6 wherein the gaseous mixture comprises nitrogen with 0.5-30% N₂ Oor NO, by volume.
 17. A method according to claim 1 wherein theexcitation of the gaseous medium is carried out at a pressure in therange of 0.1 to 50 m bar.
 18. A method according to claim 1 whereinexcitation of the gaseous medium is achieved by passing it through anelectric field produced by microwave power of at least 500 watts.
 19. Amethod according to claim 1 wherein the gaseous medium comprisesnitrogen with 0.2% by volume of SF₆.
 20. A method according to claim 18wherein the gaseous medium is excited initially by passing it at a flowrate of 500 standard cubic centimeters per minute and a pressure ofbetween 1 and 7 m bar through an electric field produced by microwavepower in the region of 500-800 watts.
 21. A method according to claim 1wherein the gaseous medium is cooled after activation.
 22. A method asclaimed in claim 1 wherein any charged species produced when activatingthe gaseous medium are removed by causing them to recombine beforeexposing the article to the activated gaseous medium.